Process and composition for coating metals

ABSTRACT

Method and composition for applying a coating to a metallic surface, the surface being immersed in an acidic aqueous coating composition comprising dispersed solid particles of an organic polymeric resinous coating-forming material and an oxidizing agent, wherein the weight or thickness of the coating formed on the surface is a function of the time the surface is immersed in the composition.

This is a continuation of application Ser. No. 904,019, filed May 8,1978, now abandoned, which is a divisional of application Ser. No.499,039, filed Aug. 20, 1974, now U.S. Pat. No. 4,104,424, which is acontinuation-in-part of application Ser. No. 113,685, filed Feb. 8,1971, now abandoned which is a divisional of application Ser. No.791,801, filed Jan. 16, 1969, now U.S. Pat. No. 3,585,084, which is acontinuation-in-part of application Ser. No. 554,336, filed June 1,1966, now abandoned.

This invention relates to the application of coatings of metallicsurfaces.

Hereinafter in this specification, as well as in the claims, the term"ferriferous" means iron, steel, and alloys of iron; the term "steel" ismeant to denote alloys of iron wherein iron is the principal ingredient,such as cold- and hot-rolled steel in which the alloying constituentscomprise minor amounts of carbon, silicon and manganese. When utilizedherein the term "zinciferous" means zinc and zinc alloys in which zincis the principal constituent, as well as galvanized surfaces, includingboth hot-dipped and electrogalvanized surfaces.

It is known that when an article of steel is immersed in an aqueousdispersion of a film-forming polymer, the thickness of the resultingcoating depends on such factors as total solids, specific gravity andviscosity. Time of immersion is not a determinative factor. For a givenaqueous dispersion, the thickness obtained after ten minutes immersionis not appreciably different from the thickness obtained after fiveseconds immersion. Further, when the article is withdrawn from theaqueous polymeric dispersion, it cannot be rinsed without removingvirtually all of the polymer from the surface, thus demonstrating a lackof adherence to the substrate. Another shortcoming of this method ofapplying a coating is that when the article is withdrawn from thecoating bath, little or no coating is formed on the edges of thearticle.

It is an object of this invention to provide a method for producingcoatings on metallic surfaces by immersing said surface in a coatingcomposition which forms a coating on the surface, the weight of thecoating being a function of the time the surface is immersed in thecomposition.

It is another object of this invention to provide an improved processand composition for forming on a metallic surface an organic resinous ornon-resinous coating wherein the coating is applied by immersing thesurface in a coating composition.

It is another object of this invention to produce organic coatings onmetallic surfaces, including particularly ferriferous and zinciferoussurfaces, of controllable thickness or coating weight, the coatingweight being a function of coating time.

It is another object of the present invention to provide a method andcomposition for applying coatings to metallic surfaces from aqueouspolymeric dispersions or solutions which coatings can be madeappreciably thicker than those obtainable heretofore by single stageoperations.

A related object of the invention is the provision of a method andcomposition for applying polymeric coatings to metallic surfaces whichrender it unnecessary to resort to multiple stage coating operations toattain a coating of the desired weight and properties.

It is an additional object of the present invention to form coatings onmetallic surfaces from aqueous polymeric dispersion or solutions, whichcoatings display appreciably improved corrosion resistance and adhesionproperties.

Still another object of this invention is the production of coatingsfrom aqueous polymeric dispersions or solutions on metallic surfaceswhich coatings are initially adherent, and thus capable of being rinsedbefore baking or drying without removing more than a superficial layerof the coating, to thereby provide increased flexibility in processingand handling.

Another object of this invention is to provide a resinous coatingcomposition and method for applying a coating to the edges of a metallicsurface.

Still another object of this invention is to provide metallic surfaceswith coatings which are continuous and free of pinholes and holidayswhen the coating is applied by immersing the surface in an aqueouscoating composition.

In accordance with this invention, it has been found that objects setforth above can be realized by immersing or dipping a metallic surfacein an acidic aqueous composition comprising an organic coating-formingmaterial and an oxidizing agent. Organic coating-forming materials whichcan be utilized in this invention are resinous materials such as, forexample, polyethylene, polyacrylics, and styrene-butadiene copolymer andnon-resinous organic materials such as, for example, fatty acids. Theorganic coating-forming ingredient can be present in the acidic aqueouscomposition either in dissolved form, in emulsified form, or indispersed form.

A preferred composition of this invention is an acidic aqueouscomposition comprising fluoride ion and an oxidizing agent selected fromthe class consisting of hydrogen peroxide and dichromate and particlesof resin dispersed in the composition, wherein the hydrogen and fluorideions are added to the composition in the form of hydrofluoric acid, andwherein the pH of the composition is within the range of about 1.6 toabout 3.8. Said preferred composition is described also in U.S. Pat. No.3,592,699 and is claimed therein.

Also in accordance with this invention, there is provided a process forapplying an organic coating-forming material to a metallic surface froman acidic aqueous carrier of said material, the process comprising:immersing the surface in the carrier; generating a precipitating agentin said carrier in the region of said surface, said agent comprising atleast in part metallic ions, preferably an ionized form of the metalcomprising the surface, and derived in part by the chemical attack ofsaid carrier on said surface; depositing the coating-forming materialfrom said carrier onto the surface by the action of said precipitatingagent to form a coating on the surface; maintaining the surface in thecarrier a period of time during at least a portion of which the coatingincreases in thickness; and withdrawing the coated surface from thecarrier.

In accordance with one aspect of this invention, it has been found thatorganic resinous and non-resinous coatings can be applied to a metallicsurface by the method comprising: dissolving ions from the metallicsurface by immersing it in an acidic aqueous composition comprising anorganic coating-forming material; precipitating the coating-formingmaterial with ions of the metallic surface in the region of the metallicsurface and the ions; maintaining the metallic surface in thecomposition as the organic coating-forming material deposits on thesurface to form a coating thereon; and withdrawing the coated surfacefrom the composition after sufficient time has elapsed to allow thecoating to build-up in thickness. By way of example, when a ferriferoussurface is immersed in an acidic aqueous coating composition of thisinvention, iron is dissolved from the surface by hydrogen ions toprovide ferrous ions in solution. The oxidizing agent can function tooxidize the ferrous ions to ferric ions. In the case where a chargedorganic coating material is present, the ferric ions function todischarge the charged organic coating-forming material, for example,negatively charged dispersed resin particles. The particles precipitateand coagulate on the surface where they form a continuous film ofthickness which is continuously increased with time. Upon withdrawal ofthe metallic surface from the composition, the surface has formedthereon a coating which is initially adherent to the metallic substrate.Thus, the coating resists being washed off the surface when the surfaceis rinsed.

With respect to the charged organic coating-forming material that isused in the composition, it is noted that there are materials which aresoluble in the composition and which inherently carry a charge. Watersoluble resins, for example, polyelectrolytes such as polyacrylic acid,are examples of such materials. Emulsified or dispersed particles caninherently carry a charge, for example, anionic and cationic materials.On the other hand, organic materials which inherently do not carry acharge and which are not ionizable, for example, nonionic materials, maynevertheless be charged due to the presence of ionizable materials inthe composition such as, for example, surfactants, which may be adsorbedon the surface of the particle. Thus, when the term "charged organiccoating-forming material" is used herein, it includes a material whichcarries a charge due to the presence of ionizable materials which areadsorbed on the surface of the particle or otherwise associatedtherewith. Nonionic colloidal materials, for example, nonionicsurfactants such as ethoxylated alkylphenols, have been deposited alsoby the process of this invention.

As mentioned above, the coating composition of the present invention iseffective in dissolving metal from the metallic surface to form ionsfrom the metallic surface in the composition, which ions are effectivein causing the organic coating-forming material to deposit and coagulateon the surface in a manner such that organic coatings which grow withtime are formed. The composition of the present invention is effectivein dissolving at least about 25 to about 60 mg/sq.ft. of the metallicsurface within about the first minute of time the metallic surface isimmersed in the coating composition. By dissolving metal in an amount ofat least about 25 to about 60 mg/sq.ft. within the first minute, it ispossible to form on the metallic surface within a relatively shortperiod of time, for example, within about 15 to about 60 seconds aresinous coating having a thickness of at least about 0.1 mil and acoating which has a greater thickness than can be obtained by immersingthe metallic surface in heretofore known latex compositions of the sameresin concentration. It should be understood that coating compositionswithin the scope of the present invention can also be effective indissolving metal of the metallic surface in amounts greater than about25 to about 60 mg/sq.ft. within the first minute of contact with themetallic surface, and that in general the thicknesses of coatings formedfrom such higher dissolution rate compositions build up at a faster ratethan those formed from compositions which dissolve about 25 to about 60mg/sq.ft., of metal within the first minute. From the above discussion,it should be understood that one of the basic aspects of the presentinvention is the capability of the composition of the present inventionof dissolving metal from the metallic surface at a rate which will causethe resin to deposit and coagulate on the metallic surface to formresinous coatings which grow with time.

The present invention, described in detail below, provides a coatingprocess which has a number of extremely important characteristics whichare not possessed by heretofore known processes. As mentionedhereinabove, heretofore known latex resin compositions can be utilizedto form resinous coatings on metallic surfaces by immersing the surfacein a bath of the composition, but the thickness of the resultant coatingis substantially the same regardless of how long the surface isimmersed. In essence, the weight or thickness of a coating that can beobtained from a particular latex resin formulation is limited when thesurface is simply immersed therein. In order to build up the thicknessof the coating, applicators have subjected metallic surfaces to amultiple stage coating operation in an effort to attain a coating of thedesired thickness. Such operation includes immersing the metallicsurface in a latex resin bath, withdrawing the surface, drying or fusing(as by heating) the coating formed thereon and then repeating theimmersion and drying steps until the thickness of the coating issatisfactory. This, of course, is a time-consuming and costly operation.Moreover, some film-forming latex resins do not readily adhere tothemselves and efforts to build up layer upon layer of the resin havebeen frustrated or special techniques must be utilized. This furtheradds to the cost of the multiple stage application process.

The present invention provides the means by which the thickness orweight of a resinous coating formed on a metallic surface from animmersion bath can be controlled by varying the time the surface isimmersed in the bath. By way of example, it is noted that in oneparticular application of the process and composition of this invention,a steel panel which was immersed for 2 minutes in a coating bath hadformed thereon a coating weighing in the range of 450 mgs./sq. ft., andthat a second steel panel immersed in the same bath for 10 minutes hadformed thereon a coating weighing in the range of 1,550 mgs./sq. ft.

It is noted that there has been developed an immersion applicationmethod by which the weight or thickness of a coating formed on ametallic surface from an aqueous polymeric dispersion can be controlledthrough the use of electricity. This method is generally referred to asthe electrocoat or electrodeposit process for painting metals. Theprocess includes the steps of immersing a metallic surface in an aqueousdispersion of resin (a latex) or an aqueous solution of resin andmaintaining the surface therein as an electric current is passed betweenthe surface to be coated and a secondary electrode. By way of example,the surface to be coated or painted is connected to a high-voltagedirect-current source and given a positive or negative charge and thetank holding the resin composition is given a charge opposite that ofthe surface to be coated. As current is passed through the system, theresin component of the aqueous composition deposits on the metal surfaceand forms a coating thereon. Benefits which are achieved by utilizingthe electrocoat or electrodeposit method, which benefits can also beachieved by use of the present invention, are film or coating thicknessor weight can be controlled, sharp edges can be coated and a uniform andcontinuous coating, that is, one which does not contain runs or sags,holidays or pinholes, can be obtained. In other words, the presentinvention can be utilized to achieve benefits provided by theelectrodeposit painting process, but with the important advantage thatthe use of electricity and equipment and control instruments required tooperate the electrocoat process is avoided.

A method of applying a coating to a metallic surface according to thisinvention is distinguishable from the heretofore mentionedelectrocoating method in that the use of electricity is obviated. Thusthe coating can form on the metallic surface unaided by the applicationof an external electrical potential to the surface. To state it anotherway the process of this invention can be carried out in a substantiallyelectrostatic field-free environment. It should be understood that thesurface may have some charge or electric potential as a result of beingimmersed in the coating composition. Thus, when the terms "externalelectrical potential" or "substantially electrostatic field-freeenvironment" are used herein, it means that the metallic surface has nocharge due to the application of an electric potential from a sourceother than the composition in which it is immersed.

The invention described herein can be utilized to coat a variety ofmetallic surfaces. Particularly good results have been obtained in thecoating of ferriferous and zinciferous surfaces.

Metallic surfaces which have thereon a previously formed coating alsocan be coated by the invention. Such previously formed coatings may beof the crystalline or amorphous types. Process and compositions forapplying such coatings are well known. By way of example, such coatingscan include those that are generally referred to as phosphates,chromates, oxalates, and oxides (anodized or chemically converted)coatings.

Continuing with the detailed description of the invention, the organiccoating-forming material, as mentioned above, may be present in theacidic aqueous composition either in dissolved form, emulsion form, orin the form of insoluble particles dispersed in the composition. Thecoating-forming material may be either a resinous compound or anon-resinous compound. (As to compositions containing non-resinouscompounds, see U.S. Pat. No. 3,776,848 of the present applicants.)Examples of non-resinous coating materials that can be utilized arefatty acids, such as, for example, stearic acid. Examples of watersoluble resinous materials are polyacrylic acid and ethylene maleicanhydride (Monsanto DX-840-12). Aqueous solutions of polyacrylic acidsare available commercially, for example, those sold under the nameAcrysol A-1 and Acrysol A-3. It is preferred to utilize an aqueouscomposition which has particles of resin dispersed in it.

In formulating a coating composition that has particles of resindispersed in it, it is preferred that the other ingredients of thecomposition be added to a latex, that is a dispersion of insoluble resinparticles in water. Latices, which are the source of the dispersed resinparticles, are readily available and those sold commercially can ofcourse be utilized. Examples of commercially available latices arePliolite 491, a styrene-butadiene latex; Catalin 1464, an acryliccopolymer latex; and Poly-em 40, a polyethylene latex. In addition tohaving dispersed therein resin solids, latices usually contain otheringredients including, for example, emulsifiers and protective colloids.The other ingredients used in the composition of this invention arepreferably added to the latices in solution form. Upon addition of theother ingredients to a latex, there is obtained a composition which canbe characterized as an acidic aqueous solution of an oxidizing agenthaving dispersed therein solid resin particles.

The amount of organic coating-forming material utilized in the acidicaqueous composition can vary over a wide range. The lower concentrationlimit is dictated by the amount of coating material needed to providesufficient material to form a coating. The upper limit is dictated bythe amount of material which can be dissolved, emulsified, or dispersedin the acidic aqueous composition.

The coating composition of this invention is acidic and thus containsacid. Any acid which in combination with the oxidizing agent causesmetal dissolution at the rate mentioned hereinabove can be used in thecomposition of the present invention. For example, the acid can be amineral or an organic acid. Typical examples of mineral acids that canbe used are sulphuric, hydrochloric, hydrofluoric, nitric, phosphoric,hydrobromic and hydroiodic. Examples of organic acids that can be usedare acetic, chloracetic, trichloracetic, lactic, tartaric andpolyacrylic acid. Examples of other acids that can be used arefluoboric, fluotitanic and fluosilicic. Hydrofluoric acid is a preferredacid.

With respect to the use of an acid such as, for example, polyacrylicacid, a resinous material of this nature which is soluble in thecomposition can be the source of not only the coating-formingingredient, but also the source of the necessary hydrogen ions. By wayof example, it is noted that polyacrylic resinous coatings have beenapplied to steel panels from an aqueous solution of polyacrylic acid andhydrogen peroxide.

It is noted also that an acid which contains an anion that functions asan oxidizing agent can be the source of not only hydrogen ion, but alsothe oxidizing agent. An example of such an acid is nitric acid.

As previously mentioned, it is preferred that the aqueous compositionhave a pH within the range of about 1.6 to about 3.8. Thus, thepreferred amounts of acids are those which impart to the composition apH within the preferred range.

The acid component dissociates, of course, to yield hydrogen ion and ananion. If it is desired to have a particular anion present in acomposition such as, for example, fluoride ion, which gives particularlygood results, and it is desired also to use an acid other than one whichcontains the particular anion, the anion can be added by way of asoluble salt containing the anion.

As mentioned above, the coating composition of the present inventioncontains also an oxidizing agent. The acid and oxidizing agentingredients of the composition play interrelated roles in formingorganic coatings which grow with time. Although an acid alone incombination with the organic coating-forming material may be effectivein causing metal dissolution rates equal to the rates mentionedhereinabove, such compositions are not effective in forming the types ofcoatings produced by the composition of the present invention whichcontains also an oxidizing agent. It is believed that the "acid only"composition results in the evolution of hydrogen gas as metal isdissolved, which prevents or deters the deposition of the coagulatingresin on the metallic surface. It has been observed that in the absenceof the oxidizing agent the resin coagulates in the composition ratherthan depositing on the metallic surface in appreciable amounts. It isbelieved also that the oxidizing agent leads to the formation of waterrather than the evolution of hydrogen gas. Another result achieved bythe use of the oxidizing agent is that the metal dissolution rate is ingeneral accelerated. Generally speaking, the metal dissolution rate fora composition containing both acid and oxidizing agent will be higherthan for a composition containing acid only.

Examples of oxidizing agents that can be used are hydrogen peroxide,dichromate, perborate, bromate, permanganate, nitrite, nitrate, andchlorate. Such oxidizing agents are oxygen-containing materials whichhave a positive reduction potential greater than that of hydrogen andfunction as oxidizing agents in the acidic aqueous composition of thepresent invention. (The reduction potential of hydrogen is zero asmeasured by a platinum electrode immersed in an acidic solution incombination with bubbling hydrogen gas. See pages 1215 to 1218 ofHandbook of Chemistry by Lange, N. A., Ninth Edition, published byHandbook Publishers, Inc., 1956 and see Chapter 1 of OxidationPotentials by Latimer, W. M., Second Edition, Prentice-Hall, Inc.,1952.) The oxygen present in the oxygen-containing oxidizing agentprovides oxygen for the formation of water from the oxidation ofhydrogen. It is believed that oxidizing agents with a reductionpotential greater than 0.77 volts (the reduction potential of theferrous-ferric couple) oxidize ferrous ions, present from thedissolution of an iron-containing metallic substrate, to ferric ionswhich are effective in causing coagulation and deposition of resin oniron-containing metallic substrates.

The oxidizing agent can be conveniently added to the composition in theform of its water soluble salt, such as for example alkali and ammoniumsalts. Particularly good results have been obtained when the oxidizingagent is one which releases oxygen in the acidic aqueous coatingcomposition. Hydrogen peroxide is such an oxidizing agent.

The preferred oxidizing agents are hydrogen peroxide and dichromate,with hydrogen peroxide being most preferred. Hydrogen peroxide can beadded conveniently to the composition in the form of a 30% aqueoussolution. As to the source of dichromate, excellent results have beenobtained by utilizing a dichromate salt, for example, calciumdichromate. However, any water soluble hexavalent chromium-containingcompound, which forms dichromate in an aqueous acidic medium can beused. For example, chromates and chromic acid can be used as the sourceof dichromate.

The amount of oxidizing agent that should be used is an amountsufficient to provide an oxidizing equivalent of at least 0.01 per literof the composition. (The term "oxidizing equivalent" when used hereinmeans the number of grams of oxidizing agent used divided by theequivalent weight of the oxidizing agent. The equivalent weight of theoxidizing agent is the gram molecular weight of the agent divided by thechange in valence of all atoms in the molecule which change valence(usually one element).) Amounts of oxidizing agents which provide anoxidizing equivalent somewhat below 0.01 can be used, but preferably theoxidizing equivalent should be at least about 0.01.

The use of an insufficient amount of oxidizing agent in combination withan acid that does not dissolve the metallic surface at the ratesmentioned hereinabove fails to produce resinous coatings which grow withtime. The use of an insufficient amount of oxidizing agent incombination with an acid that is capable of dissolving the metallicsurface at the rates mentioned hereinabove results in the evolution ofundesirable amounts of hydrogen gas which prevent or deter the formationof the desired types of coatings. Except as noted below, it appears thatthere is no critical upper limit as to the oxidizing equivalents thatare used; however, it is preferred that the oxidizing agent be presentin an amount such that the upper oxidizing equivalent value is about0.2. However, it should be understood that the oxidizing agent can beused in an amount to provide an oxidizing equivalent much higher than0.2, for example, one or more.

It is mentioned hereinabove that there appears to be no critical upperlimit as to the oxidizing equivalent that can be used. However, whenusing, as the oxidizing agent, dichromate which is known to inhibit acidattack and metal dissolution of a metallic surface, care must be takento insure that the proportion and concentrations of dichromate and acidused are such that the metal dissolution rates mentioned hereinabove areobtained. It is difficult, if not impossible, to define theconcentrations and/or proportion of dichromate and acid that under allcircumstances are effective in giving the desired metal dissolutionrates because of the numerous variables inherently present in thecomposition, including mainly the variety of acids with widely differentstrengths that can be used. However, the appropriate proportion andconcentrations can be readily determined empirically. Adichromate-containing composition which does not give the aforementionedmetal dissolution rates should have added thereto additional acid in anamount which is effective in overcoming the inhibiting effect of thedichromate which effect precludes or deters the desired metaldissolution rate.

As previously mentioned in connection with the method aspects of thisinvention, metal ions tend to be produced from the metallic surfacebeing coated. In order to inhibit or deter the presence of excessiveamounts of these ions in the composition, a chelating agent can be used.

There appears hereinafter a description of a preferred compositionwithin the scope of this invention. The preferred composition is onewhich contains a dispersed resin as the coating-forming ingredient, thesource of the resin being a latex therefore, in combination withfluoride ion and either hydrogen peroxide or dichromate as the oxidizingagent. More particularly the preferred aqueous acidic coatingcomposition comprises:

(a) about 5 g/l to about 550 g/l of resin solids dispersed in thecomposition, the source of the resin being a latex thereof;

(b) about 0.4 g/l to about 5 g/l of fluoride ion;

(c) an oxidizing agent selected from the class consisting of H₂ O₂ anddichromate, said agent being present in an amount sufficient to providefrom about 0.01 to about 0.2 of oxidizing equivalent per liter ofcomposition; and

(d) hydrogen ion in an amount sufficient to impart a pH to thecomposition of about 1.6 to about 3.8.

As will be apparent from the discussion which follows, coatingcompositions within the scope of this invention can be formulated withabove mentioned ingredients present in amounts outside of the ranges setforth above--the above described composition being a preferred one.

With respect to the resin component of the above described preferredcomposition, it is present in the composition in the form of dispersedparticles. This aqueous resin dispersion is preferably supplied as alatex.sup.(1). The latex should be stable, of course, in the presence ofthe other ingredients comprising the composition. The manifestation ofan unstable latex or one which cannot be stabilized is the dispersedresin will precipitate, flocculate or gel.

Examples of commercially available latices which can be used include thefollowing:

    ______________________________________                                        TRADENAME      REPORTED COMPOSITION                                           ______________________________________                                        Pliolite 491   Styrene-butadiene                                              Acrylene 45    Acrylic co-polymer                                             Catalin A-1464 Acrylic co-polymer                                             Catalin A-1482 Acrylic co-polymer                                             Geon 552       Polyvinyl chloride                                             Hycar 2600X 91 Acrylic co-polymer                                             Hycar 2600X 92 Acrylic co-polymer                                             Catalin A-1422 Acrylic co-polymer                                             Rhoplex HA-12  Acrylic co-polymer                                             Poly-Em 40     Polyethylene                                                   Pliovic 400    Acrylic co-polymer                                             Teflon         Tetrafluoroethylene                                            ______________________________________                                    

In accordance with the invention, coating-forming latices other thanthose listed above, may be employed in the method and compositions ofthe invention. However, the group of latices listed above has been foundto give very satisfactory and acceptable results, and for this reasonmay be regarded as the preferred set of latex resin materials for use inthe invention.

The amount of dispersed resin utilized in the coating composition canvary over a wide range. The lower concentration limit of the resin isdictated by the amount of resin needed to provide sufficient resinousmaterial to form a coating. The upper limit is dictated by the amount ofresin which can be dispersed in the acidic aqueous solution. Althoughhigher or lower amounts can be used, it is preferred that thecomposition contain from about 5 to about 550 g/l of resin. The volumeof latex utilized to provide the desired amount of resin will varydepending on the amount of resin solids dispersed in the latex, aslatices varying in solids content are of course available.

The concentration of the resin in the composition has an influence onthe weight of coating that will be obtained, other factors heldconstant. Compositions with greater amounts of a particular resin willproduce higher coating weights. For example in one series of experimentspanels were immersed for one minute in a composition that contained 1.5g/l of hydrogen peroxide, 2 g/l of fluoride ion (added as HF) anddifferent amounts of acrylic resin, the source of which was CatalinA-1316, latex. When the acrylic resin content was about 12 g/l a coatingweight of 232.2 mgs/sq. ft. was obtained, whereas when the compositioncontained in the range of about 250 g/l of acrylic resin, the weight ofthe coating was almost 1550 mgs/sq. ft.

As mentioned above, the preferred aqueous acidic coating composition ofthis invention contains fluoride ion. The optimum, preferred method ofmaking the composition acidic and adding fluoride ion comprises the useof hydrofluoric acid. This acid permits a simple means for control overpH requirements of the composition and obviates the need for introducingthe fluoride ion in the form of an alkali metal, ammonium or other salt.While coatings can be obtained by adding the fluoride in salt form, itis preferred to utilize hydrofluoric acid and avoid the use of saltswhich may give rise to undesirable cations in the coating composition orcomplicate pH adjustment. If the fluoride component is added in the formof a salt, the pH of the composition can be adjusted by the use of acidsother than hydrofluoric or in combination with hydrofluoric. Examples ofsuch acids include sulfuric, phosphoric, nitric and hydrochloric.

With respect to the fluoride ion concentration, amounts within the rangeof about 0.4 to about 5 g/l of composition (calculated as F) arepreferred. Nevertheless, higher or lower amounts can be utilized toprovide coatings wherein the coating weight builds up as a function oftime.

As noted hereinabove, the preferred pH value of the acidic coatingcomposition is within the range of about 1.6 to about 3.8. This pH maybe measured by any conventional means, the standard glass electrodemethod being conventional practice. However, due to the presence offluoride ion in the coating solution, the pH value should be determinedby rapidly observing the value obtained on initial immersion of theglass electrodes. Where the pH is permitted to fall below about 1.6, thecoating composition may tend to excessively etch the metal surface, andthe composition may become unstable. On the other hand, when the pH ofthe coating composition rises above about 3.8, the composition tends toimpart very thin coatings to the metal substrate.

The oxidizing agent used in the preferred coating composition ishydrogen peroxide or dichromate ion (Cr₂ O₇). Hydrogen peroxide is mostpreferred. The hydrogen peroxide can be added conveniently in the formof a 30% aqueous solution of hydrogen peroxide.

The dichromate constituent can be added in the form of a variety ofwater soluble hexavalent chromium-containing compounds. Examples of suchcompounds include chromic acid, potassium dichromate, magnesiumdichromate, potassium chromate and sodium chromate. Any water solublehexavalent chromium-containing compound, which in an aqueous acidicmedium forms dichromate can be used. Preferred sources of the dichromateingredient are dichromates, for example calcium dichromate. Particularlygood results have been obtained by utilizing an aqueous solution ofchromic acid and a calcium salt, for example calcium carbonate. Inaddition, particularly good results have been obtained by adding to thecomposition an aqueous solution made up from potassium dichromate andcalcium acetate. It is preferred also that the source of dichromate beadded to the latex used in the form of an aqueous solution of thehexavalent chromium-containing compound.

The preferred amount of oxidizing agent is an amount sufficient toprovide an oxidizing equivalent of about 0.01 to about 0.2 in one literof the composition. Somewhat lesser amounts of the oxidizing agent whichprovide an oxidizing equivalent outside of the lower value can beutilized also. The upper equivalent value is not critical and can bemuch higher, provided that when utilizing dichromate, the guidelinesdiscussed hereinabove be followed. For example, resinous coatings havebeen obtained when the amount of hydrogen peroxide used provided anoxidizing equivalent in excess of one. It has been observed that whendichromate is utilized as the oxidizing agent in amounts to provideoxidizing equivalents in the higher range, then higher amounts offluoride should be used--for example 31/2 to 5 grams, when thedichromate equivalent is within the range of about 0.1 to about 0.2.

As to particularly preferred amounts of the oxidizing agent, thereshould be utilized about 0.3 to about 3.0 g/l of hydrogen peroxide(approximately 0.02 to 0.2 equivalents) and from about 1 g/l to about 2g/l of dichromate (approximately 0.03 to 0.055 equivalents). However,when an aqueous solution made up from chromic acid and calcium carbonateor when an aqueous solution made up from potassium dichromate andcalcium acetate is used, then lower amounts of dichromate can beutilized and thicker coatings can be obtained, for example, about 0.735g/l to about 0.95 g/l of dichromate (approximately 0.02 to 0.03equivalents).

The above described preferred composition can be utilized to goodadvantage to produce quality coatings the thickness of which can becontrolled by the time a metallic surface is immersed therein.

There follows a description of conditions under which a coating may beapplied in the practice of this invention.

The time of immersion of a metallic surface in the coating compositionof this invention may vary from as little as 30 seconds to as much as 10minutes or even longer. However, it has been found that while coatingweights increase with prolonged treating times, maximum coating weightsseem to be realized within about 10 minutes time, so that longerexposure of the metal surfaces to the action of the coating compositionsgenerally do not yield correspondingly heavier coating weighs. Thismatter of coating weights is also dependent to some extent upon the typeof coating-forming material employed, so that in any particular instancepreliminary coating weight determinations may be run in order toascertain the type of coatings which are likely to be obtained with aparticular system. In the interest of economy, suffice it to say that itis preferred to operate utilizing coating cycles of from about 1 toabout 3 minutes duration.

As was pointed out above, the coating weight, for a particular coatingcomposition and type of metal surface being treated, tends to increase,up to a maximum, as the time of treatment is increased. Once theoperating characteristics of a particular coating system have beenascertained, this fact can be exploited to provide a convenient, readilyvariable control parameter for securing the desired coating weight. If alight coating is desired, a short treating time can be employed, andwhen a heavy coating is desired, the treating time can be lengthened.This advantage is unavailable to those using prior treating methods,because the coating weight obtained under those prior methods is not, asa practical matter, a function of time.

This feature of the invention is illustrated by the following data. Whena treating composition conforming to Example 1, below was used to coatsteel panels under the treating conditions set out in detail in thediscussion of Example 1, it was found that after two minutes of exposurethe steel panels had a coating weighing 467 mgs./sq. ft. After fourminutes of exposure, the coating weight obtained was 813 mgs./sq. ft.,and after ten minutes exposure the coating weight increased to 1563mgs./sq. ft. Further lengthening of the exposure time beyond ten minutesdid not result in an increase in coating weight.

With respect to bath temperature, this is preferably operated anywherefrom ambient temperature, that is, from about 20° C., up to about 40° C.If the coating bath temperatures are permitted to rise much in excess ofabout 40° C. it has been found that coating weights begin to decrease,so that if heavier coatings are desired they will not be obtained byraising the temperature--other factors held constant. Nevertheless,coatings can be produced when the temperature of the composition is inexcess of about 40° C. Obviously, temperatures which render thecomposition unstable should be avoided. Since the coatings obtained atambient temperature are completely satisfactory, it is preferred tooperate at this temperature so as to obviate the necessity ofmaintaining heated bath compositions, and thus obtain a reduction incoating costs. Of greater importance, however, is bath stability, whichis at its maximum at room temperature. It has been noted that withrespect to latices, these are more stable at room temperature.

Nevertheless, some advantages can be obtained by immersing the metallicsurface in a heated coating composition. With all factors held constantexcept the temperature of the coating bath, it has been found thathigher weight coatings can be obtained as the temperature of thecomposition is raised. The coating weight begins to fall off as thetemperature exceeds a certain limit, which limit will vary depending onthe type of coating-forming material utilized in formulating the coatingcomposition.

It is preferred that relative motion be maintained between the coatingcomposition and the metallic surface immersed therein. This may beaccomplished, for example, by stirring the composition with a mixer orby moving the surface in the composition. By maintaining relative motionbetween the surface and the composition, heavier or thicker coatings canbe obtained. By way of example, it is noted that in one experimentwherein a metallic surface was moved in the composition, there wasobtained a coating that weighed almost ten times as much as a coatingformed on a surface that was simply immersed in the composition with norelative motion between it and the composition being maintained.

Following treatment of metallic surfaces in the acidic aqueous coatingcomposition of this invention, such surfaces may, if desired, be rinsedwith water. A water rinse is not required.

It has been found that the corrosion resistance or resin-coated metallicsurfaces can be improved by rinsing with water followed by rinsing withan aqueous rinse solution containing chromium, for example a dilutesolution of chromic acid. There are many types of chromium-containingrinse compositions available and many of them are sold commercially. Apreferred chromium rinse composition is one obtained when a concentratedaqueous solution of chromic acid is treated with formaldehyde to reducea portion of the hexavalent chromium. This type of rinse composition,which is described in U.S. Pat. No. 3,063,877 to Schiffman, containschromium in its hexavalent state and reduced chromium in aqueoussolution. By way of example, such an aqueous rinse composition cancomprise a total chromium concentration within the range of about 0.15g/l (expressed as CrO₃) to about 2 g/l, wherein from about 40-95% of thechromium is in its hexavalent state and the remainder of the chromium isin its reduced state. Other chromium rinses that can be utilized to goodadvantage are dilute solutions of chromic acid and solutions of sodiumdichromate.

Whether or not a final water rinse is employed, the coating should beallowed to dry to evaporate water. In the case of a resinous coating, itshould be allowed to fuse. This serves to render the coating continuous,thereby improving its resistance to corrosion and adherence to theunderlying metal surfaces.

The conditions under which the drying operation is carried out dependsomewhat upon the type of resin employed. Fusion characteristics ofcoatings formed from the various types of resins which can be utilizedin practicing the invention are known and the drying conditions bestutilized for a coating formed from a specific resin can be selected onthe basis of previous experience. Some the resins which are suitable foruse in the invention do not require high temperatures for fusion, andfor these resins air drying at ambient temperatures can be used. Dryingcan be accelerated by placing the coated surface in a heatedenvironment. Most of the resinous coatings require heated drying stages,or baking, in order to fuse the resin and to insure that the desiredcorrosion resistance is obtained. In summary, while the drying operationcan be performed at room temperature under some circumstances, it isgenerally preferred that it be done by oven drying or baking.

When a heated environment is used, the drying or fusion stage may becarried out at temperatures above 110° C., and preferably from 120° to150° C. However, it is to be understood that whatever temperature isultimately employed will depend, at least in part, on the particularresin utilized. For example, coatings formed from a polyethylene latexare satisfactorily fused within a temperature range of from about 110°C. to about 140° C.; tetrafluoroethylene coatings require appreciablyhigher temperatures. Since the resinous materials are organic, they willtend to degrade if extremely high drying temperatures are employed, andit is, of course, preferred that such temperatures be avoided. Thisfactor presents no difficulty in the operation of the invention, sinceadequate drying or fusion without degradation is easily obtainable byoperation within the temperature ranges set out above.

Drying temperature is also partially dependent upon the time cyclesemployed. Where relatively short drying cycles are used, i.e., from 30seconds to two or three minutes, higher oven temperatures are needed. Ifthe drying cycle is of appreciably longer duration, such as, forexample, 10 to 15 minutes, then generally lower temperatures can beutilized. Suffice it to say, the choice of drying conditions will bedictated by considerations of the type of coating formed and dryingcycles contemplated.

Although coating compositions prepared in accordance with the teachingsof this invention are capable of producing films on metallic surfaceswhich have outstanding qualities with respect to both adhesion andcorrosion resistance, it has also been discovered that still furtherenhancement of corrosion resistance qualities can be realized if acoalescing agent is incorporated into the coating composition.

Among the many coalescing agents which are available, it is preferredthat ethylene glycol monobutyl ether, commonly known as butylCellosolve, be used. This particular agent demonstrates completecompatibility in various proportions in the coating composition of thisinvention, and imparts a high degree of adhesion with respect to bondingpolymeric films. Examples of other coalescing agents that can be usedare hexylene glycol, diethylene glycol monoethyl ether acetate,diethylene glycol monobutyl ether acetate and ethylene glycol monobutylether acetate.

With respect to the amount of coalescing agent used, it is preferredthat from about 5 to about 30 grams (per liter of composition) of thisagent be employed to realize the enhanced corrosion resistant andadhesion properties; nevertheless, higher or lower amounts can beutilized.

As an aid in assuring thorough wetting of the metallic surface duringtreatment, it is sometimes preferable to incorporate into the coatingcomposition a small quantity of a wetting agent, such as up to about0.15% by weight of the total composition, over and above that which maybe present in the source of the coating-forming material, for example, alatex. Use of wetting agents is preferred practice where the metallicsurface to be treated is not thoroughly cleaned, since such agentspermit wetting of the metallic substrate with removal of some or all ofthe contaminants present thereon. Preferably non-ionic or anionic typewetting agents are used; they provide satisfactory degrees of wettingwhen incorporated into the coating composition of this invention.Examples of wetting agents that can be utilized are alkyl phenoxypolyethoxy ethanol and sodium salts of alkylaryl polyether sulfonate.

If desired, the coating composition of the present invention may beformulated so as to provide decorative or aesthetic effects upon treatedmetallic surfaces. When applied to metallic surfaces, the colors of thecoatings produced tend to vary depending on a number of factors,including for example, the conditions under which the coatings are driedor fused and the coating-forming material used. Variations in the colormay be realized by adding to the composition commonly usedwater-dispersible pigments, such as for example, phthalocyanine blue,phthalocyanine green, carbon black or quinacridone red. Generally, thesepigments provide excellent color variations with no sacrifice in coatingquality. Any pigment that is compatible with the composition and doesnot make it unstable can be used.

The amount of pigment which may be employed will depend, as is wellknown in the art, upon the depth or degree of hue desired. It should benoted that when dispersible pigments are used, it is advantageous toemploy a small quantity of a wetting agent, in accordance with the abovediscussion, to aid in dispersing the pigment and maintaining it insatisfactory dispersion.

It has been observed that metallic surfaces coated according to thisinvention have formed thereon an organic-inorganic coating, with theinorganic coating being sandwiched between the surface of the metal andthe organic coating. The organic coating comprises the coating-formingmaterial used in the coating composition. The nature of the inorganiccoating has been difficult to characterize; however, when coatingferriferous surfaces, there has been found some evidence which wouldindicate that the inorganic coating is an oxide of the metal beingcoated, for example iron oxide. It can be said that this inventionprovides a process for applying simultaneously an inorganic coating andan organic coating to a metal substrate. Whatever the exact nature ofthe coatings, it is apparent that their corrosion resistant and adherentproperties can be vastly superior to coatings produced by previousmethods. The outstanding results that can be obtained from theutilization of the present invention will be apparent from a number ofthe examples set forth hereinafter.

In order to demonstrate the present invention there is presented below aseries of examples showing the use of various types and grades of latexresins in aqueous solutions of hydrogen peroxide and fluoride ion inaccordance with the teachings of this invention. The hydrogen peroxidewas added in the form of a 30% aqueous solution of hydrogen peroxide andthe hydrofluoric acid, the source of the fluoride ion, was added in theform of a 70% hydrofluoric acid solution. Also added to some of theseexamples are the various other additives which have been found to besuitable for use in this invention.

EXAMPLE 1

    ______________________________________                                        Component             Parts by Wt.                                            ______________________________________                                        Styrene-butadiene resin*                                                                            50 grams                                                Hydrogen peroxide     1.5 grams                                               Hydrofluoric acid     2.1 grams                                               Water, to make        1 liter                                                 ______________________________________                                         *Source  Pliolite 491 latex                                              

Steel panels, previously cleaned in a conventional alkali metal silicatesolution, were immersed in the bath of Example 1 for two minutes at 25°C. After removal from the bath these panels were divided into two sets,one set being dried in an oven at 140° C. for 10 minutes, the other setbeing subjected to water rinsing prior to oven drying as above. Coatingthicknesses for both sets of panels averaged 0.55 mil, no differencesbeing detectible between the rinsed and the unrinsed panels. Asmentioned, the composition attacks and dissolves metal as the coating isformed. It was found that steel panels coated with the above compositionexperienced a weight loss of about 24 mg/sq. ft. within the first minuteof contact with the composition.

Salt spray corrosion tests (ASTM B-117-61) were run on representativepanels from both sets treated in accordance with the above procedure.Completely satisfactory results were obtained after 16 hours exposure.

Adhesion tests were also run on panels from each set utilizing thefollowing procedures which are commonly used in the testing of paints:

(1) Cross-Hatch Test

This test is an alternate method for testing paint adhesion. The paintedsurface is scribed with parallel lines approximately 1/16" apart and cutthrough to bare metal. Duplicate lines are scribed at right angles tomake a cross-hatch pattern.

Scotch brand cellophane tape is pressed smoothly over the scribed area.After several seconds, the tape is pulled back rapidly so that the tapeis turned back upon itself approximately 180° from its original pressedposition.

Results are reported in the degree of failure noted: e.g. none, slight,moderate or heavy loss of paint.

(2) Impact Test

The test panel is impacted by a falling 1/2" ball with a given forcemeasured in inch-pounds, deforming the test specimen. After impact, thedeformed surface is inspected for loose or cracked paint, usually on thereverse side of the impact, and rated descriptively. The degree ofdeformation, and accordingly the severity of any given force of impact,is dependent upon the thickness of the specimen. Normally, specimensheavier than 16 gage (0.0625") are not used, for the maximum impactavailable--160 inch-pounds--causes little deformation to these heavysections.

Results from these adhesion tests were excellent for both the waterrinsed and the unrinsed panels.

EXAMPLE 2

Clean galvanized steel panels were immersed in an aqueous dispersioncontaining the following constituents, utilizing a coating temperatureof 25° C. and a coating cycle of two minutes.

    ______________________________________                                        Component             Parts by Wt.                                            ______________________________________                                        Styrene-butadiene resin*                                                                            50 grams                                                Hydrogen peroxide     1.1 grams                                               Hydrofluoric acid     1.26 grams                                              Water, to make        1 liter                                                 ______________________________________                                         *Source  Pliolite 491 latex                                              

Following water rinsing and oven baking at 140° C. for ten minutes thesecoated panels were subjected to salt spray corrosion testing (ASTMB-117-61) and to Cross-hatch and Impact adhesion tests (as describedabove in Example 1). Results of these tests were completely acceptable.

EXAMPLE 3

In order to illustrate the improved adhesion results flowing from theprocess of the present invention, as compared with the use of a latexresin alone, clean steel panels were immersed in an aqueous dispersioncontaining the following constituents utilizing a coating temperature of22° C. and a two minute immersion cycle.

    ______________________________________                                        Component            Parts by Wt.                                             ______________________________________                                        Polyethylene resin*  5 grams                                                  Hydrogen peroxide    1.1 grams                                                Hydrofluoric acid    1.26 grams                                               Water, to make       1 liter                                                  ______________________________________                                         *Source  Polyem 40 latex                                                 

Following treatment the steel panels were baked at 140° C. for 10minutes.

Other clean steel panels were immersed in an aqueous dispersioncontaining 5 grams per liter of polyethylene resin, the source of whichwas also Poly-em 40, but containing no hydrogen peroxide or hydrofluoricacid. These panels were also baked at 140° C. for 10 minutes.

Panels resulting from these two separate treatments were then subjectedto the Cross-hatch and the Impact adhesion tests, as described above inExample 1. Results showed that the panels treated in the dispersions ofthis invention displayed excellent adhesion properties, showing nofailure, while the panels treated in the aqueous dispersion ofpolyethylene without the use of hydrogen peroxide and hydrofluoric aciddemonstrated complete failure in both adhesion tests.

EXAMPLE 4

In order to illustrate the enhanced corrosion resistance derived fromthe use of a coalescing agent, steel panels were immersed in an aqueousdispersion containing the following constituents and utilizing animmersion cycle of one minute at 30° C.

    ______________________________________                                        Component             Parts by Wt.                                            ______________________________________                                        Acrylic co-polymer resin*                                                                           50 grams                                                Hydrogen peroxide     1.2 grams                                               Hydrofluoric acid     1.3 grams                                               Water, to make        1 liter                                                 ______________________________________                                         *Source  Hycar 2600X92 latex                                             

Following the immersion cycle, the steel panels were immediatelysubjected to a drying cycle of 10 minutes at 140° C.

Butyl Cellosolve (ethylene glycol monobutyl ether), in the amount of 18grams/liter, was added to the bath of Example 4, and steel panels wereimmersed for the same one minute cycles at 30° C. as used above. Aftertreatment these panels were immediately dried at 140° C. for 10 minutes.

Salt spray corrosion tests (ASTM B-117-61) were run on both sets ofpanels. After 16 hours exposure incipient corrosion was observed onthose panels treated in the bath which contained no butyl Cellosolve,whereas steel panels treated in the bath which did contain butylCellosolve were completely free of any traces of corrosion even after114 hours testing.

Additional examples were run to illustrate various features of thisinvention. These are reported below in Table 1 along with the corrosiontest results and the adhesion test results, all determined in accordancewith the procedures set forth herein above in Example 1. Reference inthe Table to "ASTM B-117-61 signifies the salt spray corrosion test andthe hours reported represent the duration of the test wherein nocorrosion was observed on the treated metal panels. The references to"Impact" and "Cross-Hatch" represent adhesion tests as described above.

                                      TABLE I                                     __________________________________________________________________________           Aqueous Coating Composition                                                                             Test Results.sup.(1)                                Latex                                                                         Used And                  ASTM                                         Ex.                                                                              Metal                                                                             Resin Resin                                                                             F.sup.-                                                                          H.sub.2 O.sub.2                                                                   Other    B-117-61 Cross-                              No.                                                                              Used                                                                              Therein                                                                             g/l g/l                                                                              g/l Additives                                                                              hours                                                                              Impact                                                                            Hatch                               __________________________________________________________________________    5  Steel                                                                             Catalin                                                                             50  1.5                                                                              1.0 5 g/l Aurasperse                                                                        16  Excel.                                                                            No                                         A-1464,          W 6014, Phthalo   Failure                                    acrylic          Green (Harshaw                                               Co-              Chemical Co.)                                                polymer          (as a pigment)                                        6  Steel                                                                             Poly-em                                                                             550 0.4                                                                              3.0 20 g/l butyl                                                                           144  "   No                                         40,              Cellosolve as     Failure                                    poly-            coalescing                                                   ethylene         agent                                                 7  Zinc                                                                              Pliolite                                                                            5   5  3.0 20 g/l butyl                                                                           114  "   No                                         491,             Cellosolve as     Failure                                    styrene-         coalescing                                                   butadiene        agent                                                 8  Steel                                                                             Hycar 250 5  0.3 0.1 g/l poly-                                                                           18  "   No                                         2600X-92         ethoxylated       Failure                                    acrylic          alkyl phenol                                                 co-              as wetting                                                   polymer          agent                                                 9  Steel                                                                             Catalin                                                                             25  3  1.5 5 g/l butyl                                                                            138  "   No                                         A-1464,          Cellosolve as     Failure                                    acrylic          coalescing                                                   co-              agent                                                        polymer                                                                10 Zinc                                                                              Geon  12.5                                                                              1.5                                                                              2.0 25 g/l butyl                                                                           124  "   No                                         552              Cellosolve as     Failure                                    polyvinyl        coalescing                                                   chloride         agent                                                 11 Zinc                                                                              Rhoplex                                                                             12.5                                                                              2.5                                                                              2.5 30 g/l butyl                                                                            22  "   No                                         HA-12            Cellosoilve as    Failure                                    acrylic          coalescing                                                   co-poly.         agent                                                 12 Steel                                                                             Teflon.sup.(2)                                                                      50  2  0.75          16  "   No                                         Disp.                              Failure                                    No. 30                                                                 __________________________________________________________________________     .sup.(1) All aqueous dispersions had pH values between 1.6 and 3.8. All       coatings were applied at 25° C. using 5 minute immersion cycles.       All coated panels were baked at 140° C., except the Teflon coated      panel which was baked at temperatures in the range of about 315° C     to about 370° C., using 10 minute baking cycles.                       .sup.(2) As supplied by Chemplast, Inc., Newark, N.J.                    

Set forth hereinafter are additional examples which show various facetsof the invention. Unless otherwise indicated, appropriate amounts of HF(70%) and H₂ O₂ (30%) were utilized in formulating the coatingcompositions of the examples to provide the stated amounts of thefluoride and hydrogen peroxide components. The source of the resincomponent is identified by the latex used. Appropriate amounts of thelatex were used to provide the stated amount of resin.

Set forth in Table II below are examples which strikingly illustratethat the coating composition of this invention can be utilized toprovide a metal surface with a coating, the weight of which isinfluenced by the time the surface is immersed in the composition. Twodifferent coating compositions were used in gathering the results setforth in Table II. One composition contained about 12 g/l of acrylicresin (Catalin A-1316 latex), 2 g/l of fluoride, and 1.5 g/l of hydrogenperoxide. The other composition contained the same amounts of fluorideand hydrogen peroxide, but instead of acrylic resin, it contained about50 g/l of polyethylene (Poly-Em 40 latex). The results set forth inTable II were obtained by immersing steel panels in the compositions forthe various time periods set forth and determining the coating weightsafter the coatings were dry. The coating compositions were at ambienttemperature when the steel panels were immersed therein.

                  TABLE II                                                        ______________________________________                                        Ex.       Time of Immersion                                                                           Coating Weight                                        No.       in Minutes    (mgs./sq. ft.)                                        ______________________________________                                                  Acrylic                                                             13        1             238.2                                                 14        2             467.4                                                 15        3             679.2                                                 16        4             813.6                                                 17        5             916.2                                                 18        10            1563.6                                                          Polyethylene                                                        19        1             579.0                                                 20        3             1306.2                                                21        5             1596.0                                                ______________________________________                                    

As mentioned hereinabove, the temperature of the coating composition hasan influence on the weight of coating produced on the metallic surface.This is illustrated in Table III below. The composition used inobtaining the results set forth in the table contained about 12 g/l ofacrylic resin (Catalin A-1316 latex), 2 g/l of fluoride, and 1.5 g/l ofhydrogen peroxide. Steel panels were immersed in the composition for 3minutes at the various temperatures set forth in the table. The weightsof the coatings produced on the panels were determined after thecoatings were dried.

                  TABLE III                                                       ______________________________________                                                 Temperature (°F.)                                             Ex.      of             Coating Weight                                        No.      Coating Composition                                                                          (mgs./sq. ft.)                                        ______________________________________                                        22       75              764.4                                                23       92.5            850.2                                                24       100            1070.4                                                25       110            1040.4                                                26       120            1022.4                                                ______________________________________                                    

It can be seen from the table that significant increases in the coatingweights were obtained as the temperature was raised to about 100° F.,but that as the temperature of the coating composition was raised above100° F. (38° C.) the coating weights began to decrease for thisparticular composition.

There is set forth in Table IV below the weight of coatings formed onsteel panels from aqueous coating compositions having different pHvalues. The composition contained approximately 50 g/l of acrylic resin,the source of which was Catalin A-1316 latex, 2 g/l of fluoride and 1.5g/l of hydrogen peroxide. The pH of this composition, which was 2.37,was varied and adjusted upwardly by adding 1.0 N NaOH to it. Between pHadjustments a panel was immersed in the composition. The weights of thecoating formed on the panels from the composition are set forth inExamples 27, 28 and 30 of Table IV. Another coating composition,substantially the same in all respects as the one described above, wasformulated, but its pH, which was 2.4, was varied and lowered by addingthereto concentrated HNO₃. Panels were immersed in the compositionbetween pH adjustments. The coating weights formed on the panels are setforth in Examples 29, 31 and 32 of the table. The time of immersion forall panels was one minute.

                  TABLE IV                                                        ______________________________________                                        Ex.        pH of      Coating Weight                                          No.        Composition                                                                              (mgs./sq. ft.)                                          ______________________________________                                        27         4.0         48.6                                                   28         3.6        160.8                                                   29         2.4        573.0                                                   30         2.37       643.8                                                   31         2.0        265.8                                                   32         1.6         49.2                                                   ______________________________________                                    

The next two examples are illustrative of the treatment of coatedmetallic surfaces, subsequent to their withdrawal from the coating bath,with chromium-containing rinse compositions.

EXAMPLE 33

A steel panel was immersed in a coating composition containing 50 g/l ofacrylic resin (Catalin A-1316 latex), 2 g/l of fluoride, and 1.5 g/l ofhydrogen peroxide for 3 minutes. Upon withdrawal from the compositionthe coated panel was rinsed with an aqueous solution containinghexavalent chromium and reduced chromium. (The composition was preparedaccording to the method described in aforementioned U.S. Pat. No.3,063,877.) The concentrations of the hexavalent and the reducedchromium were each about 0.5 g/l (expressed as CrO₃). The coated panelwas treated for 30 seconds with the rinse composition which had atemperature of 130° F. After the rinsing step was completed, the coatedpanel was baked to complete fusion of the coating.

EXAMPLE 34

A steel panel was treated in the same manner as set forth in Example 33above, except that the rinse composition used in this example containedpolyacrylic acids in addition to hexavalent chromium and reducedchromium. (This type of composition is disclosed in U.S. Pat. No.3,185,596 to Schiffman.) The rinse composition contained 10 g/l of Cr⁺⁶and a like amount of reduced chromium (expressed as CrO₃) and 4.1 g/l ofpolyacrylic acids (source-Acrysol-A-1, a solution of water solublepolyacrylic acids).

After each of the panels of Examples 33 and 34 above were rinsed, theywere subjected to a salt spray test (ASTM B-117-61). Another steel panelwhich was coated and baked in the same way as those of Examples 33 and34, but which was not rinsed, was also subjected to the salt spray test.The results of the salt spray tests showed that the rinsed panels hadmuch better corrosion resistance properties than the unrinsed panel,with the panel of Example 33 outperforming slightly the panel of Example34.

The next two examples are illustrative of the use of a coatingcomposition containing dichromate as the oxidizing agent and build-up ofcoating thickness as a function of time of immersion of a metallicobject in the composition.

EXAMPLE 35

There was formulated an aqueous coating composition which containedabout 50 g/l of styrene-butadiene resin (Pliolite 491 latex), 2 g/l offluoride, and 1.1 g/l of dichromate (the source of the dichromate was 2mls. of an aqueous solution prepared from 1.02 gs. of chromic acid and0.19 g. of CaCO₃). A steel panel was immersed in the coating compositionfor 5 minutes. Upon withdrawal of the coated panel from the compositionit was rinsed with tapwater and then dried at 140° C. for 10 minutes.Subsequent to the drying step, it was determined that the thickness ofthe coating was 0.7 mil. The coated panel was subjected to a salt spraytest (ASTM B-117-61). The result of the test was 168 hours.

EXAMPLE 36

The same coating composition and procedure set forth in Example 35 wasutilized to coat a steel panel except that the panel was allowed toremain in the coating composition for 10 minutes instead of 5 minutes.The coating had a thickness of 1.5 mils and the result of the salt spraytest was 360 hours.

From examples 35 and 36, it can be seen that the panel of Example 36,which was immersed for a period of time twice as long as that of Example35, had a coating more than twice as thick as the panel of Example 35and corrosion resistant properties more than twice as good.

Exemplary coating compositions showing the use of various types ofresins in combination with dichromate and fluoride are set forth inTable V. Each composition contained about 50 g/l of the resin identifiedin the table, 1.1 g/l of dichromate (added as the previously describedaqueous solution of chromic acid and calcium carbonate), and 2 g/l offluoride (added as 70% HF acid). Each of the compositions had a pH of3.2 except the composition of Example 37 which had a pH of 3.4. Steelpanels were immersed in the compositions and the thicknesses of thecoatings obtained were measured. The coated panels were subjected to theASTM B-117-61 salt spray corrosion test. The test results as well as thethicknesses of the coatings are set forth in the table. There is alsoset forth the latex which was utilized as the source of the resin.

                  TABLE V                                                         ______________________________________                                                Latex Used      Coating   ASTM                                        Ex.     and             Thickness B-117-61                                    No.     Resin Therein   (mils)    (hours)                                     ______________________________________                                        37      Pliolite 491    0.6       242                                                 Styrene-butadiene                                                     38      Acrylene 45     0.5       190                                                 Acrylic co-polymer                                                    39      Catalin A-1464  1.0       190                                                 Acrylic co-polymer                                                    40      Catalin A-1482  0.3       88                                                  Acrylic co-polymer                                                    41      Chemigum 235    0.7       232                                                 Butadiene                                                                     Acrylonitrile                                                         42      Geon 552        1.0       232                                                 Polyvinyl chloride                                                    43      Hycar 2600X-91  1.1       88                                                  Acrylic co-polymer                                                    44      Hycar 2600X-92  0.6       88                                                  Acrylic co-polymer                                                    45      Catalin A-1422  0.2       24                                                  Acrylic co-polymer                                                    46      Rhoplex HA-12   0.3       16                                                  Acrylic co-polymer                                                    47      Poly-Em 40      0.6       16                                                  Polyethylene                                                          48      Pliovic 400     <0.1      16                                                  Acrylic co-polymer                                                    49      Poly-Em 20      1.0       16                                                  Polyethylene                                                          ______________________________________                                    

Set forth in Table VI below are exemplary coating compositions showingsome of the sources which can be used to provide the dichromatecomponent of the composition of this invention, as well as the use ofvarying amounts of dichromate. Each composition contained about 50 g/lof styrene-butadiene resin (Pliolite 491 latex), 2 g/l of fluoride addedas 70% hydrofluoric acid and dichromate as indicated in Table VI. It isnoted that the sources of dichromate, which were added to thecomposition in solution form, are also given in the table, as well asthe pH of certain of the compositions. It is noted also that thecompositions of Examples 56 to 60 contained calcium acetate and thatthose of Examples 62-70 contained calcium carbonate. The amounts ofthese ingredients are set forth in the table. Steel panels were immersedin the compositions for 5 minute periods and the thicknesses of thecoatings obtained were measured after the coated panels were rinsed withtap water and dried at 140° C. for 10 minutes. The coated panels werethen subjected to the ASTM B-117-61 salt spray corrosion test. Thesetest results, as well as the thickness of the coatings, are set forth inthe table.

                                      TABLE VI                                    __________________________________________________________________________                  Conc. of                                                                           Conc.                                                                    Cr.sub.2 O.sub.7 In                                                                Of                                                            Source     Coating                                                                            Ca  pH  Coating                                                                             ASTM                                         Ex.                                                                              of         Comp.                                                                              Salt                                                                              of  Thickness                                                                           B-117-61                                     No.                                                                              Dichromate g/l  g/l Comp.                                                                             (mil.)                                                                              (hours)                                      __________________________________________________________________________    50   K.sub.2 Cr.sub.2 O.sub.7                                                               0.735                                                                              --  NR* 0.1   16                                           51   "        1.102                                                                              --  3.4 0.3   43                                           52   "        1.47 --  NR  0.25  16                                           53   "        2.205                                                                              --  "   0.13  16                                           54   "        2.94 --  "   0.03  16                                           55   MgCr.sub.2 O.sub.7                                                                     1.08 --  3.3 0.25  24                                                K.sub.2 Cr.sub.2 O.sub.7                                                 56   and      0.735                                                                              0.2 3.5 0.4   215                                               Ca acetate                                                                    K.sub.2 Cr.sub.2 O.sub.7                                                 57   and      "    0.4 3.5 0.6   215                                               Ca acetate                                                                    K.sub.2 Cr.sub.2 O.sub.7                                                 58   and      "    0.6 3.6 0.5   215                                               Ca acetate                                                                    K.sub.2 Cr.sub.2 O.sub.7                                                 59   and      "    0.8 3.6 0.4   215                                               Ca acetate                                                                    K.sub.2 Cr.sub.2 O.sub.7                                                 60   and      "    1.0 3.6 0.4   215                                               Ca acetate                                                               61   chromic acid                                                                           1.5  --  3.0 0.25  NR                                                chromic acid                                                             62   and      0.47 0.1 3.0 0.05  16                                                Ca carbonate                                                                  chromic acid                                                             63   and      0.94 0.19                                                                              3.0 0.1   16                                                Ca carbonate                                                                  chromic acid                                                             64   and      "    "   3.4 0.6   242                                               Ca carbonate                                                                  chromic acid                                                             65   and      "    "   3.5 0.6   263                                               Ca carbonate                                                                  chromic acid                                                             66   and      1.17 0.23                                                                              3.4 0.6   263                                               Ca carbonate                                                                  chromic acid                                                             67   and      1.27 0.26                                                                              3.4 0.8   382                                               Ca carbonate                                                                  chromic acid                                                             68   and      1.42 0.28                                                                              3.3 0.3   16                                                Ca carbonate                                                                  chromic acid                                                             69   and      1.65 0.33                                                                              3.3 0.3   16                                                Ca carbonate                                                                  chromic acid                                                             70   and      1.89 0.38                                                                              3.5 0.1   16                                                Ca carbonate                                                             __________________________________________________________________________

Set forth in Table VII below are exemplary coating compositions showingsome of the sources which can be used to provide the acid component ofthe composition of this invention. Each of the compositions of theexamples contained 50 g/l of styrene-butadiene resin (Pliolite 491latex), 1.5 g/l of H₂ O₂ and the acid set forth in the table. The acidwas added to the composition in amounts sufficient to impart to thecomposition the pH values stated in the table.

                  TABLE VII                                                       ______________________________________                                                    Acid       pH                                                     Ex.         Added to   of                                                     No.         Composition                                                                              Composition                                            ______________________________________                                        71          Phosphoric 2.6                                                    72          Hydrochloric                                                                             2.45                                                   73          Sulfuric   2.75                                                   74          Acetic     2.88                                                   75          Lactic     2.85                                                   76          Tartaric   2.8                                                    ______________________________________                                    

Steel panels were immersed in each of the compositions of Examples 71-76and upon withdrawal of the panels from the compositions resinouscoatings were formed thereon. It will be appreciated from thecompositions set forth in the above table that a variety of types andkinds of acids can be utilized in the compositions of this invention.

The next example is illustrative of a composition containing a mixtureof acids.

EXAMPLE 77

There was formulated 1 liter of aqueous composition containing 50 g ofstyrene-butadiene resin (Pliolite 491 latex), 5 ml of 21% hydrofluoricacid (1 g. of F), 5 ml of 75% phosphoric acid and 1.5 g of H₂ O₂. Thiscomposition was utilized to provide a steel panel with an excellentresinous coating.

The next example shows the use of an aqueous coating compositioncomprising a solution of dissolved resin coating material to form aresinous coating on a metallic surface according to the invention.

EXAMPLE 78

To a beaker, there was added 100 mls. of an aqueous solution ofpolyacrylic acids (source-Acrysol A3--25 wt. % polyacrylic acids havinga molecular weight less than 150,000), 5 mls. of 30% H₂ O₂ (1.5 g) andsufficient water to make 1 liter of the composition. A steel panelimmersed in the composition and withdrawn therefrom had formed thereon apolyacrylic resin coating.

With respect to the above example, it is noted that the source of theacid component for the coating composition described therein was thepolyacrylic acid ingredient which was also the source of theresin-coating ingredient.

The next example shows the application to a metallic surface of acoating comprising two different resins by the use of this invention.

EXAMPLE 79

To 1 liter of the composition of Example 78, there was added 100 mls. ofan acrylic latex (Catalin A-1316 latex--approximately 50 wt. % resinsolids) to provide 1100 mls. of a composition having dispersed thereinparticles of acrylic resin and approximately 25 g of polyacrylic acidsdissolved in the composition. A steel panel immersed in the compositionand withdrawn therefrom had formed thereon a resinous coating which wasmade up of the acrylic resin of the latex and the resin derived from thepolyacrylic acids component.

Set forth in Table VIII below are examples referring to coatingcompositions which illustrate some of the oxidizing agents which can beutilized in the composition of this invention. Each of the compositionscontained 2 g/l of fluoride (added as hydrofluoric acid), 50 g/l of theresin identified in the table and the oxidizing agent (and amountsthereof) set forth in the table.

                  TABLE VIII                                                      ______________________________________                                                                       Conc. of                                                                      Oxidizing                                      Ex.    Resin and    Oxidizing  Agent                                          No.    Latex Used   Agent      (g/l of Comp.)                                 ______________________________________                                        80     Styrene-     Ammonium   10.3                                                  Butadiene    persulfate                                                       Pliolite 491                                                           81     Styrene-     Sodium     6.9                                                   Butadiene    perborate                                                        Pliolite 491                                                           82     Acrylic      Potassium  4.7                                                   Catalin A-1316                                                                             permanganate                                              83     Acrylic      Sodium     6.2                                                   Catalin A-1316                                                                             nitrite                                                   84     Acrylic      Potassium  4.5                                                   co-polymer   nitrate                                                          Catalin A-1464                                                         85     Acrylic      Sodium     1.9                                                   co-polymer   chlorate                                                         Catalin A-1464                                                         86     Acrylic      Sodium     2.3                                                   co-polymer   bromate                                                          Catalin A-1464                                                         ______________________________________                                    

Steel panels were immersed in each of the compositions of the examplesset forth in Table VIII. Upon withdrawal of the panels, it was foundthat each of the compositions was effective to impart to the panel aresinous coating. From these examples it will be appreciated that avariety of oxidizing agents can be utilized in the practice of thisinvention.

The next example is illustrative of the use of one ingredient whichfunctions as an oxidizing agent and a source of acid for thecompositions of this invention.

EXAMPLE 87

There was formulated 1 liter of aqueous composition containing 50 gs. ofacrylic resin (Catalin A-1464 latex) and sufficient nitric acid toimpart to the composition a pH of 2.5. This composition was utilized toform a resinous coating on a steel panel which was immersed in thecomposition.

The next example shows a non-resinous organic coating composition andits use to form a coating on a metallic surface immersed thereinaccording to the invention.

EXAMPLE 88

1 liter of aqueous composition containing 50 grams of stearic acid, 2grams of fluoride (added as hydrofluoric acid) and 1.5 grams of hydrogenperoxide was prepared. This composition also contained 20 mls of ananionic surfactant and a few mls of a nonionic surfactant. The anionsurfactant was Tergitol Anionic P-28 sodium di-(2-ethylhexyl) phosphate.The nonionic surfactant was Triton N100 (an ethoxylated nonyl phenol).The stearic acid, the coating-forming ingredient, was present in thecomposition in emulsified form. A steel panel was immersed in thecomposition for 5 minutes. Upon withdrawal of the panel it was foundthat there had formed thereon a coating of stearic acid, which coatingcould not be rinsed off with running tap water.

As mentioned above, the coating composition of the present invention iscapable of dissolving metal from the metallic surface to form metal ionsin an amount sufficient to cause the dispersed solid resin particles todeposit on the surface in a manner such that the weight of coating is afunction of the time the surface is immersed in the composition. Thenext group of examples is illustrative of the effect that compositionswithin the scope of the present invention have on dissolving metal fromthe metallic surface. The dissolution was determined by calculating theweight losses of a group of metallic surfaces immersed for differentperiods of time in the coating composition. Coating weights were alsodetermined. The following coating composition was used.

EXAMPLE 89

    ______________________________________                                        Components       Parts by Weight                                              ______________________________________                                        styrene-butadiene latex                                                                         100 g                                                       (Pliolite 491 latex)                                                          hydrogen peroxide                                                                               2.4 g                                                       hydrofluoric acid                                                                              0.42 g                                                       water, to make   1 liter                                                      ______________________________________                                    

The pH of the composition was 2.95. A group of unpolished cold-rolledsteel panels, 3"×4", was weighed before they were immersed in the abovecoating composition. The panels were immersed in the coating compositionfor the periods of time set forth in Table IX below. After withdrawingthe panels from the coating composition, they were rinsed under runningtap water and then baked for 10 minutes at 170° C. The coated panelswere then weighed. In order to determine the weight losses of thepanels, another group of panels was subjected to the same procedure asdescribed above, but immediately after the coated panels were withdrawnfrom the composition, they were treated to remove therefrom the coating.This was accomplished by scrubbing the coated panel with a soft nylonbrush and then immersing the scrubbed panels in a chromic acid solution(50 g/l) at 160° F. for 5 minutes. This removed inorganic coating thatwas formed. Thereafter, the panels were scrubbed again under runningwater and then washed with acetone to remove any traces of organiccoating or particles that remained. The panels were then dried andweighed. Subtracting this weight from the original weight of the panelsgave the weight of the panels after they were subjected to the coatingcomposition. The weight losses, along with the weights of the coatingsformed on the panels are set forth in Table IX below.

                  TABLE IX                                                        ______________________________________                                                     Coating Weight,                                                                            Weight Loss,                                        Time, in seconds                                                                           in mg/sq. ft.                                                                              in mg/sq. ft.                                       ______________________________________                                         0           146.4        not determined                                      10           not determined                                                                             not determined                                      20           not determined                                                                             not determined                                      30           156.6         7.8                                                40           210.3        17.4                                                50           224.4        23.4                                                60           241          30.6                                                ______________________________________                                    

With reference to the above table, it is noted that the panel identifiedas being immersed in the composition for 0 second was immersed in thecomposition and immediately withdrawn therefrom. It can be seen thatbetween 0 and 30 seconds there was little appreciable gain in thecoating weight and very little loss in weight of the metal panel,representing very little dissolution thereof. Calculations show thatover the period of 60 seconds, the amount of coating deposited wasalmost 65 percent greater than that deposited when the panel wasimmersed in the composition and immediately withdrawn therefrom.

Tests like the tests described above in connection with Example 89 werealso run with the following coating composition which containedhydrochloric acid instead of hydrofluoric acid.

EXAMPLE 90

    ______________________________________                                        Component        Parts by Weight                                              ______________________________________                                        styrene-butadiene latex,                                                      Pliolite 491 latex                                                                             100 g                                                        hydrogen peroxide                                                                               2.4 g                                                       hydrochloric acid                                                                               2.2 g                                                       water, to make   1 liter                                                      ______________________________________                                    

The pH of the composition was 2.7. The same procedure as described abovein connection with Example 89 was followed, except that after the panelswere withdrawn from the compositions, they were water rinsed byimmersing the coated panels in a container of water for 15 seconds andmoving them therein with a to and fro motion. The results are set forthin Table X below.

                  TABLE X                                                         ______________________________________                                                     Coating Weight,                                                                            Weight Loss,                                        Time, in Seconds                                                                           mg/sq. ft.   mg/sq. ft.                                          ______________________________________                                         0           146.2        not determined                                      10           145.2         4.2                                                20           178.8         7.8                                                30           238.2        19.8                                                40           255.6        24.6                                                50           291.6        27.9                                                60           320.4        31.8                                                ______________________________________                                    

With respect to Table X above, calculations show that there was anincrease in the coating weight of almost 120 percent during the periodof time of one minute. It can be seen also that appreciable gains incoating weight were achieved when the loss in weight of the metal panelwas in the range of about 20 to about 25 mg/sq. ft.

As mentioned above, when utilizing an oxidizing agent such asdichromate, if sufficient acid is not present, the dichromateconstituent will function to prevent or deter metal dissolution at thedesired rate of at least about 25 to about 60 mg/sq. ft. within thefirst minute of contact time. Examples of types of compositions in whichthe dichromate constituent is present with acid in combination withdispersed particles of resin and which compositions do not function in amanner such that there is sufficient dissolution of the metallic surfaceto form the types of coatings of the present invention are disclosed invarious previously issued patents. In general, the patents disclosecompositions in which the amount of dichromate is too high for theamount of acid used. The next group of comparative examples areillustrative of this.

COMPARATIVE EXAMPLE A

The composition of this example corresponds closely to the compositionof Example 2 shown in U.S. Pat. No. 2,562,118 to Osdal. The compositioncontained 194 grams of a teflon dispersion (latex) containing about 46wt. % solids, 1 gram of CrO₃ and 1 gram of H₃ PO₄. A steel panelimmersed in this composition for one minute experienced a weight loss ofonly about 8 mg/sq. ft. The composition was not effective in depositingon the metal panel a coating which grew with time.

COMPARATIVE EXAMPLE B

The composition of this example corresponds to Example 1 of French Pat.No. 1,295,687. The composition contained the following.

    ______________________________________                                        Components         Parts by Weight                                            ______________________________________                                        polyvinyl acetate latex,                                                      Elvacet 84-1100 (about                                                        55 wt. % solids)   100 grams                                                  sodium dichromate dihydrate                                                                       30 grams                                                  phosphoric acid solution,                                                                         20 grams                                                  water              to make 1 liter                                            ______________________________________                                    

A steel panel immersed in the above composition experienced a weightloss after one minute of 1.8 mg/sq. ft. The composition was ineffectivein depositing on metal panels resin coatings which grew with time.

COMPARATIVE EXAMPLE C

The composition of this example corresponded basically to Example 28 ofU.S. Pat. No. 3,185,596 to Schiffman, except that Rhoplex AC490, a latexcontaining acrylic resin, was substituted for EDX-1 latex, unavailable.The composition contained 1 wt. % of the mixed chromium compoundsdescribed in the patent and 21.8 g/l of Rhoplex AC490 (about 46 wt. %solids). A clean steel panel immersed in the composition for one minuteexperienced a weight gain of 9.6 mg/sq. ft. The composition wasineffective in depositing on the steel panel a resinous coating whichgrew with time.

COMPARATIVE EXAMPLE D

The composition of this example corresponds to Example 1 of U.S. Pat.No. 3,053,692 to Pocock except that Elvacet 84-1100 was substituted forPolyco 453, not available. The composition contained 100 g/l of Elvacet84-1100 and 2 g/l of sodium dichromate dihydrate. A steel panel immersedin this composition for one minute experienced no weight loss. Thecomposition was ineffective in depositing on the metal panel a resinouscoating which grew with time.

A comparison of the dissolution rates of the ferriferous surfacessubjected to the compositions of Comparative Examples A to D with thoseattained in the use of the compositions of Examples 89 and 90 of thepresent invention shows clearly that the latter are capable ofdissolving the ferriferous surface to a much greater extent than theformer and form distinctly different kinds of resinous coatings. Asnoted above, the increase in amounts of the resinous coatings formed bythe use of the compositions of Examples 89 and 90 is appreciable whenthe weight losses of the ferriferous surfaces are about 25 mgs/sq. ft.,the dissolution of the surfaces being caused by the composition andresulting in the release of ferrous ions. Depending on the resin usedand the type of metallic surface being coated, the dissolution rateneeded to form the desired coatings may be higher than about 25 mg/sq.ft. within the first minute, for example, up to about 60 mg/sq. ft.within the first minute. This can readily be determined empirically andcompositions effective to accomplish this can be readily formulated fromthe teachings hereinabove. Indeed compositions which dissolvesubstantially greater amounts of the metallic surface than 60 mg/sq. ft.within the first minute can be formulated and effectively used.

Form the above description, it can be seen that the coating compositionof the present invention can be formulated from a variety of differentacids, oxidizing agents and resins to give the desired dissolution rateswhich are effective in forming resinous coatings which grow with time.Any resin which is capable of being dispersed in the form of small solidparticles in the aqueous medium of the composition can be used in thecomposition. Success has been attained when using latices containinganionic dispersing agents for the resin particles (Catalin A-1464 latexbeing an example) and those containing nonionic dispersing agents forthe resin particles (Pliolite 491 latex being an example). Additionalexamples of latices that can be used are Rhoplex MV-1 and Rhoplex AC-61(each containing an acrylic resin and sold by Rohm and Haas Company) andtwo-part polyurethane-forming resin latices which are mixed prior to useand sold as Nelthane 300A and Nelthane 300B by Northeastern LaboratoriesCo., Inc. From the above, it will be appreciated that both condensationand addition polymers can be used as the resin.

The coating composition of the present invention can be used to coatmetallic surfaces that can be used in a wide variety of applications.The coatings can be aesthetic and/or functional in nature and can serveas final finishes or as primers. Examples of applications are: coatedautomotive parts including interior parts, underhood parts and exteriorparts; coated metal furniture and bookcases; coated industrial andhousehold appliances and fixtures; coated cooking wear, particularlywear coated with polytetrafluoroethylene; coated tubing and pipes;coated structural beams and facing panels; coated radio speakers; coatedmetal frames and doors; coated metal fences and polytetrafluoroethylenecoated lock assemblies and hinges. Such parts can be coated in a mannersuch that coatings are uniform and continuous and do not contain sags,pinholes or bare spots.

We claim:
 1. An acidic aqueous coating composition having a pH of about1.6 to about 3.8 for use in coating a ferriferous surface comprising anacidic aqueous solution of at least about 0.01 oxidizingequivalent/liter of an oxidizing agent which contains oxygen and whichhas a positive reduction potential greater than that of hydrogen, andabout 5 to about 550 g/l of solid particles of a coating-formingpolymeric resinous material dispersed throughout the composition, saidcomposition being effective to chemically attack, in the absence of anexternal electrical potential, a ferriferous surface immersed therein todissolve therefrom metal in an amount of at least about 25 to about 60mg/sq. ft. within the first minute of immersion to release ions of saidmetal and sufficient to cause said resinous material to deposit on saidferriferous surface in the form of an initially adherent coating whichincreases in weight or thickness the longer the time said surface isimmersed in said composition.
 2. A composition according to claim 1wherein said resinous material is selected from the group consisting ofstyrene-butadiene, acrylonitrile-butadiene, polyethylene, acrylic,tetrafluoroethylene, polyvinyl chloride and urethane resins.
 3. Acomposition according to claim 1 wherein said oxidizing agent isselected from the group consisting of hydrogen peroxide, dichromate,perborate, bromate, permanganate, nitrite, nitrate and chlorate.
 4. Acomposition according to claim 1 wherein said acid is selected from thegroup consisting of hydrofluoric, sulfuric, hydrochloric, hydrofluoric,nitric, phosphoric, hydrobromic, hydroiodic, acetic, chloroacetic,trichloroacetic, lactic, tartaric and polyacrylic.
 5. A compositionaccording to claim 1 wherein said oxidizing agent is hydrogen peroxide.6. A composition according to claim 1 wherein said oxidizing agent isdichromate.
 7. A composition according to claim 1 wherein said acid ishydrofluoric.
 8. A composition according to claim 1 wherein the sourceof said resinous material is a latex.
 9. A composition according toclaim 8 including a wetting agent in an amount over and above that whichis present in said latex.
 10. A composition according to claim 1including a coalescing agent.
 11. A composition according to claim 10wherein the amount of said coalescing agent is about 5 to about 30 g/l.12. A composition according to claim 11 wherein said coalescing agent isethylene glycol monobutyl ether.
 13. A composition according to claim 7wherein the amount of hydrofluoric acid is such that the concentrationof fluoride ion is about 0.4 to about 5 g/l of composition.
 14. Acomposition according to claim 1 wherein the oxidizing agent is presentin an amount such that it has an oxidizing equivalent of about 0.01 toabout 0.2 per liter of composition.
 15. A composition according to claim1 wherein said acid is phosphoric acid or hydrochloric acid.
 16. Acomposition according to claim 2 wherein the oxidizing agent is presentin an amount such that it has an oxidizing equivalent within the rangeof about 0.01 to about 0.2 oxidizing equivalent per liter ofcomposition.
 17. A composition according to claim 1 wherein theoxidizing agent has a reduction potential greater than 0.77 volt.
 18. Anacidic aqueous coating composition for use in coating a metallic surfacecomprising an acidic aqueous solution of an oxidizing agent whichcontains oxygen and which has a positive reduction potential greaterthan that of hydrogen, and an emulsion of a coating-forming polymericresinous material dispersed throughout the composition, wherein theingredients of the composition are present in amounts such that saidcomposition will chemically attack, in the absence of an externalelectrical potential, a metallic surface immersed therein to dissolvetherefrom metal in an amount of at least about 25 to about 60 mg/sq. ft.within the first minute of immersion to release ions of said metal andsufficient to cause said resinous material to deposit on said metallicsurface in the form of an initially adherent coating which increases inweight or thickness the longer the time said surface is immersed in saidcomposition.
 19. A composition according to claim 18 wherein saidmetallic surface is a ferriferous surface and wherein said ions areferrous ions.
 20. A composition according to claim 18 wherein saidmetallic surface is a zinciferous surface and wherein said ions are zincions.
 21. A composition according to claim 19 wherein the pH of thecomposition is about 1.6 to about 3.8.
 22. A composition according toclaim 1 including pigment.